The present invention relates generally to an integrated circuit (IC) design, and more particularly to a system of bandgap reference circuit that is capable of below 1 volt operations and designed for providing other ICs with a reference voltage.
Voltage reference is a necessary functional block for the operation of mixed-mode and analog integrated circuits (ICs) such as data converters, phase lock-loops (PLL), oscillators, power management circuitries, dynamic random access memory (DRAM), flash memory, and much more. A voltage reference must be, at least inherently, well-defined and insensitive to temperature, power supply and load variations. The resolutions of the ICs mentioned above, such as the data converters, are limited by the precision of its reference voltage over the circuit's supply voltage and operating temperature ranges. The bandgap reference voltage is required to exhibit both high power supply rejection and low temperature coefficient, and is probably the most popular high performance voltage reference used in ICs today. IC design is now predominated by low power, low voltage objectives, making complementary metal-oxide-semiconductor (CMOS) the technology of choice.
An early attempt for the solution is a conventional bandgap reference circuit that uses conventional bipolar technology to create a stable low reference voltage at around 1.2 volts. This conventional bandgap reference circuit is designed to provide a stable reference voltage at a targeted operation point, i.e. 1.2 volts. However, a zero-current state is also a stable operating point, and the reference voltage may stay at the zero-current state even after the current of the bandgap reference circuit is built up. Therefore, this convention bandgap reference circuit is typically equipped with an additional start-up circuit. The start-up circuit is designed to provide a start-up current to initiate the current of the bandgap reference circuit to be built up. Once the current of the bandgap reference circuit is built up, the start-up current is turned off and the bandgap reference circuit will provide a stable reference voltage at the targeted operation point.
However, recent IC design typically requires sub-1 volt operation regions, thereby rendering conventional systems as discussed above not so satisfactory. While there exists other conventional bandgap reference circuits that can operate below 1 volt, there are still start-up issues. While start-up issues can be overcome by equipping these conventional circuits with start-up circuits, the existence of the interface between these conventional circuits and the start-up circuits often makes these conventional circuits unreliable.
Therefore, it is desirable to design a new bandgap reference circuit without start-up problems that can also operate at below 1 volt.